1. Field of the Invention
The present invention relates to an improved Gauss type lens system and more particularly to an improved Gauss type lens system having six groups which consists of seven elements to be used with a single lens reflex camera to provide a relatively fast, large aperture lens systems.
2. Description of the Prior Art
Gauss type lens systems or modifieid Gauss type lens systems have been generally used as a relatively large aperture ratio lens system, since field curvature can be favorably corrected with the Gauss lens design. A problem exists however relating to flare caused by off axial pencil rays since this aberration is difficult to be corrected when the aperture ratio is increased in the Gauss or modified Gauss type lens systems. Another problem relates to the back focal distance of the Gauss or modified Gauss type lens systems. Setting the back focal length in the lens system in the range of 50 to 60% is the normal design condition to maximize the solution of various aberrations. However, a back focal length of this magnitude is relatively unfavorable for a lens system that is to be used with a single lens reflex camera due to the requirement of providing sufficient back focal distance for the movable mirror located behind the system. In efforts to increase this back focal distance, the Gauss or modified Gauss type lens systems have been generally designed so that a relatively large refractive power is distributed in the lens groups at the image side of the aperture stop. This design option, however, is unfavorable in the correction of flare relating to the off axial pencil rays and also to the correction of field curvature.
As a result of these inherent problems in the Gauss design, it is extremely difficult to provide a large aperture ratio with sufficient back focal distance while at the same time correcting the various aberrations that exist. Generally the maximum back focal distance that the prior art has been able to achieve is 70% of the focal length of the system and to date there are no known lens systems of a Gauss or modified Gauss type wherein the back focal length reaches 80% of the focal length of the system. Generally with extremely high speed lens systems such as that of F No. 1.2, most of the Gauss or modified Gauss type lens systems have back focal distances less than 70% of the focal length of the system. An example of a relatively high speed modified Gauss type lens system with an angle of 46.degree. can be seen in U.S. Pat. No. 3,738,736.
Various other examples of modified Gauss type lens systems of a six group, seven element design have been suggested such as those disclosed in U.S. Pat. No. 3,874,771; U.S. Pat. No. 3,451,745; U.S, Pat. No. 3,851,953; U.S. Pat. No. 3,984,155, U.S. Pat. No. 3,743,387; and U.S. Pat. No. 3,938,884. Additionally the German Patent Application Publications DAS Nos. 1277580, DOS 2607710, and DOS 2347262 are cited of interest. Finally, the Japanese Patent Publication No. 48-10083, Japanese Patent Publication No. 41-17176, Japanese Patent Publication No. 47-54013, Japanese Patent Publication No. 49-23891, Japanese Utility Model Publication No. 47-19025 and Japanese Application laid open to public inspection No. 51-148421 also disclose a six group seven lens element modified Gauss type lens systems.
The present invention provides a novel generic design of six group, seven lens element improved Gauss type lens system which is not disclosed by any of the lens system in the aforementioned prior art patent publication as can be readily perceived from the comparison between those lens systems and the conditions of the present inventive lens systems which are hereinafter defined.